Quick drop valves are commonly used in hydraulic control circuits for bulldozer blades or the like in which the blade is allowed to free-fall to ground level under the force of gravity. Some of the fluid expelled from the hydraulic cylinders which control blade elevation is diverted by the quick drop valves to the expanding ends of the hydraulic cylinders to supplement the pump flow thereto. Without any type of quick drop valve, the expanding ends of the hydraulic cylinders cavitate quite badly. Since the cavitated ends of the cylinders have to be filled with fluid from the pump after the blade comes to rest on the ground a considerable time lag occurs before sufficient downward force can be applied to the blade for penetrating the ground. The use of quick drop valves minimizes the cavitation and thus reduces the time lag.
The duration of the time lag depends upon the efficiency of the quick drop valve which is determined by the amount of expelled fluid that the quick drop valve diverts back to the expanding side of the cylinders. That amount is dependent upon how quickly the quick drop valve moves to the quick drop position in a free-fall situation and the percentage of the expelled fluid that the quick drop valve diverts back to the expanding ends once it is in the quick drop position.
The known quick drop valves are moved to and retained in the quick drop position by differential pressure generated by the expelled fluid passing through a fixed triggering orifice once the flow rate of the expelled fluid exceeds a predetermined rate. The size of the fixed orifice usually dictates both how quickly sufficient differential pressure is generated to move the valve to the quick drop position and how much of the expelled fluid can be diverted to the expanding ends of the hydraulic cylinders. One of the problems encountered with the use of the fixed orifice is that the fluid being directed to the hydraulic cylinders to raise the blade also passes through that orifice. If the size of the orifice is reduced to a size for maximum efficiency in the quick drop mode, it restricts the flow from the pump to the hydraulic cylinders in the raise mode thereby limiting the speed at which the blade can be raised. Thus, the size of the triggering orifice is normally dictated by the rate of fluid flow from the pump to the cylinder in the raise mode such that maximum efficiency of the quick drop valve cannot be realized.
Heretofor the efficiency of the quick drop valves have been such that if a single quick drop valve was used to handle the flow from two or more hydraulic cylinders, the duration of the time lag to fill the cylinders at the ground level increased. If a quick drop valve was used for each cylinder, the overall cost of the control circuit increased. A larger single quick drop valve of the current design could possibly handle the combined flow of two or more cylinders but would increase the cost of the valve, the time lag, and implement drift.
It would be desirable to have a quick drop valve constructed so that its efficiency is such that a single quick drop valve could handle the fluid flow from two or more hydraulic cylinders without increasing the duration of the time lag or restrict fluid flow to the cylinders in the raise mode and which can be built for considerably less than the cost of the separate currently available quick drop valves required to handle the same flow. In one mode of operation, the blade is allowed to free fall from the raised position and then suddenly stopped before the blade reaches the ground to shake loose any material that might be adhering to the blade. Thus, it is desirable for the quick drop valve to be capable of being shifted from the quick drop position to the non-quick drop position at any point in the free-fall to provide this function.
The present invention is directed to overcoming one or more of the problems as set forth above.